Re: Performance analysis of Linux Kernel Markers 0.20 for 2.6.17

[Mathieu Desnoyers <compudj@krystal.dyndns.org>]

* Nicholas Miell (nmiell@comcast.net) wrote:
> On Sun, 2006-10-01 at 11:33 -0400, Mathieu Desnoyers wrote:
> > * Nicholas Miell (nmiell@comcast.net) wrote:
> > > On Sat, 2006-09-30 at 23:42 -0400, Mathieu Desnoyers wrote:
> > > > * Nicholas Miell (nmiell@comcast.net) wrote:
> > > > > 
> > > > > Has anyone done any performance measurements with the "regular function
> > > > > call replaced by a NOP" type of marker?
> > > > > 
> > > > 
> > > > Here it is (on the same setup as the other tests : Pentium 4, 3 GHz) :
> > > > 
> > > > * Execute an empty loop
> > > > 
> > > > - Without marker
> > > > NR_LOOPS : 10000000
> > > > time delta (cycles): 15026497
> > > > cycles per loop : 1.50
> > > > 
> > > > - With 5 NOPs
> > > > NR_LOOPS : 100000
> > > > time delta (cycles): 300157
> > > > cycles per loop : 3.00
> > > > added cycles per loop for nops : 3.00-1.50 = 1.50
> > > > 
> > > > 
> > > > * Execute a loop of memcpy 4096 bytes
> > > > 
> > > > - Without marker
> > > > NR_LOOPS : 10000
> > > > time delta (cycles): 12981555
> > > > cycles per loop : 1298.16
> > > > 
> > > > - With 5 NOPs
> > > > NR_LOOPS : 10000
> > > > time delta (cycles): 12983925
> > > > cycles per loop : 1298.39
> > > > added cycles per loop for nops : 0.23
> > > > 
> > > > 
> > > > If we compare this approach to the jump-over-call markers (in cycles per loop) :
> > > > 
> > > >               NOPs    Jump over call generic    Jump over call optimized
> > > > empty loop    1.50    1.17                      2.50 
> > > > memcpy        0.23    2.12                      0.07
> > > > 
> > > > 
> > > > 
> > > > Mathieu
> > > 
> > > What about with two NOPs (".byte 0x66, 0x66, 0x90, 0x66, 0x90" - this
> > > should work with everything) or one (".byte 0x0f, 0x1f, 0x44, 0x00,
> > > 0x00" - AFAIK, this should work with P6 or newer).
> > > 
> > > (Sorry, I should have mentioned this the first time.)
> > > 
> > 
> > Hi,
> > 
> > The tests I made were with : 
> > #define GENERIC_NOP1    ".byte 0x90\n"
> > #define GENERIC_NOP4        ".byte 0x8d,0x74,0x26,0x00\n"
> > #define GENERIC_NOP5        GENERIC_NOP1 GENERIC_NOP4
> > 
> > Now with the tests you ask for :
> > 
> > * Execute an empty loop
> > - 2 NOPs ".byte 0x66, 0x66, 0x90, 0x66, 0x90"
> > NR_LOOPS : 100000
> > time delta (cycles): 200190
> > cycles per loop : 2.00
> > cycles per loop for nops : 2.00-1.50 = 0.50
> > 
> > - 1 NOP "0x0f, 0x1f, 0x44, 0x00, 0x00"
> > NR_LOOPS : 100000
> > time delta (cycles): 300172
> > cycles per loop : 3.00
> > cycles per loop for nops : 3.00-1.50 = 2.50
> > 
> > 
> > * Execute a loop of memcpy 4096 bytes
> > - 2 NOPs ".byte 0x66, 0x66, 0x90, 0x66, 0x90"
> > NR_LOOPS : 10000
> > time delta (cycles): 12981293
> > cycles per loop : 1298.13
> > cycles per loop for nops : 1298.16-1298.13=0.03
> > 
> > - 1 NOP "0x0f, 0x1f, 0x44, 0x00, 0x00"
> > NR_LOOPS : 10000
> > time delta (cycles): 12985590
> > cycles per loop : 1298.56
> > cycles per loop for nops : 0.43
> > 
> 
> To summarize in chart form:
> 
>               	JoC	JoCo	2NOP	1NOP
> empty loop	1.17	2.50	0.50	2.50
> memcpy		2.12	0.07	0.03	0.43
> 
> JoC 	= Jump over call - generic
> JoCo	= Jump over call - optimized
> 2NOP	= "data16 data16 nop; data16 nop"
> 1NOP	= NOP with ModRM
> 
> I left out your "nop; lea 0(%esi), %esi" because it isn't actually a NOP
> (the CPU will do actual work even if it has no effect, and on AMD64,
> that insn is "nop; lea 0(%rdi), %esi", which will truncate RDI+0 to fit
> 32-bits.)
> 
> The performance of NOP with ModRM doesn't suprise me -- AFAIK, only the
> most recent of Intel CPUs actually special case that to be a true
> no-work-done NOP.
> 
> It'd be nice to see the results of "jump to an out-of-line call with the
> jump replaced by a NOP", but even if it performs well (and it should,
> the argument passing and stack alignment overhead won't be executed in
> the disabled probe case), actually using it in practice would be
> difficult without compiler support (call instructions are easy to find
> thanks to their relocations, which local jumps don't have).
> 

Hi,

Just to make sure we see things the same way : the JoC approach is similar to
the out-of-line call in that the argument passing and stack alignment are not
executed when the probe is disabled.

Mathieu

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